Geometry Nodes: Expose vertex normals as an attribute

This attribute exposes mesh vertex normals as a `vertex_normal`
attribute for use with nodes. Since the normal vector stored in
vertices is only a cache of data computable from the surrounding faces,
the attribute is read-only. A proper error message for attempting to
write this attribute is part of T85749. A write-only normal attribute
will likely come later, most likely called `corner_normal`.

The normals are recomputed before reading if they are marked dirty.
This involves const write-access to the mesh, protected by the mutex
stored in `Mesh_Runtime`. This is essential for correct behavior after
nodes like "Edge Split" or nodes that adjust the position attribute.

Ref T84297, T85880, T86206

Differential Revision: https://developer.blender.org/D10541

1 files changed, 3 insertions, 2 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_join_geometry.cc b/source/blender/nodes/geometry/nodes/node_geo_join_geometry.cc
index 4e15f232934..9dce52c072d 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_join_geometry.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_join_geometry.cc
@@ -222,8 +222,9 @@ static void join_components(Span<const MeshComponent *> src_components, Geometry
   dst_component.replace(new_mesh);
 
   /* Don't copy attributes that are stored directly in the mesh data structs. */
-  join_attributes(
-      to_base_components(src_components), dst_component, {"position", "material_index"});
+  join_attributes(to_base_components(src_components),
+                  dst_component,
+                  {"position", "material_index", "vertex_normal"});
 }
 
 static void join_components(Span<const PointCloudComponent *> src_components, GeometrySet &result)